Method for growing group-III nitride semiconductor heterostructure on silicon substrate

ABSTRACT

The present invention provides a method for growing group-III nitride semiconductor heteroepitaxial structures on a silicon (111) substrate by using a coincidently matched multiple-layer buffer that can be grown on the Si(111) substrate. The coincidently matched multiple-layer buffer comprises a single-crystal silicon nitride (Si 3 N 4 ) layer that is formed in a controlled manner by introducing reactive nitrogen plasma or ammonia to the Si(111) substrate at a suitably high temperature. Then, an AlN buffer layer or other group-III nitride buffer layer is grown epitaxially on the single-crystal silicon nitride layer. Thereafter, the GaN epitaxial layer or group-III semiconductor heteroepitaxial structure can be grown on the coincidently matched multiple-layer buffer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a semiconductor structure,and more particularly to a method for growing a group-III nitridesemiconductor heteroepitaxial structure on a silicon substrate.

2. Description of the Prior Art

The structure of semiconductor light-emitting diode (LED) comprises asubstrate, a light emitting structure, and a pair of electrode forpowering the diode. The substrate can be opaque or transparent. Lightemitting diodes that are based on gallium nitride compounds generallycomprise: a transparent, insulating substrate, e.g., a sapphiresubstrate. To overcome the difficulty of the substantial latticemismatch between an insulating substrate, e.g., a sapphire substrate,and GaN compound semiconductor, it is a common practice to provide athin buffer layer or nucleation layer on the sapphire substrate, whichis followed by a layer on which an LED structure is grown. Growth ofsingle crystals on insulating substrates has been studied for manyyears. Early works included growth of both silicon and III-V compoundsemiconductors on a variety of insulating substrates including sapphire.In these studies it was determined that use of nucleation or bufferlayers reduces the occurrence of imperfections and the tendency towardstwinning in the thicker layer grown thereon.

Group III nitride semiconductors [GaN (gallium nitride), InN (indiumnitride), AlN (aluminum nitride), and their alloys] have become thematerials of choice for many optoelectronic applications, especially inthe areas of fully-color or white-light light emitting diodes (LEDs) andblue laser diodes (LDs). Some scientists and engineers have evenpredicated that group-III nitrides will become all-around semiconductorsbesides their already-commercialized applications in optoelectronics. Atpresent, the major barrier for widespread applications of nitrides isthe lack of perfectly lattice-matched substrates for epitaxial growth.Sapphire (Al₂O₃) and silicon carbide (SiC) are two most popular choicesas growth substrates. But, besides the large lattice mismatch, theinsulating nature of sapphire renders the processing of nitride devicesmore difficult and costly. On the other hand, the high price and limitedsize of silicon carbide also make the widespread GaN-on-SiC applicationsdifficult. GaN-on-Si epitaxial technology represents an interestingalternative, which can eventually integrate the existing Si-basedmicroelectronic technology and the novel functionalities provided by thegroup-III nitrides.

For GaN-on-Si heteroepitaxy, the AlN buffer layer approach yields thebest results reported in the literature, leading to the demonstration oflight-emitting diodes on Si. However, the mutual solubility of Al and Siis very high at the AlN buffer-layer growth temperature (about 820° C.vs. eutectic temperature 577° C.). Therefore, the inter-diffusion of Aland Si at the interface is severe, resulting in high unintentionaldoping levels in the epilayer and the Si substrate as well as thedegradation in the film structural and optical quality.

On the other hand, It has been found that an amorphous orpolycrystalline SiN_(x)[silicon nitride (Si₃N₄) or silicon subnitride]layer can be formed by intentional or unintentional nitridation of thesilicon substrate surface during the first stage of the group-IIInitride growth. Moreover, Si₃N₄ is known to be an effective diffusionbarrier material. However, this amorphous or polycrystalline SiN_(x)layer is prone to cause detrimental effects on the properties of grownGaN films on the Si substrate since it is not possible to grow ahigh-quality crystalline film on a amorphous or polycrystalline surface.Therefore, it has been a common practice in the growth ofgroup-III-nitrides film on the silicon substrate to avoid the formationof an amorphous or polycrystalline SiN_(x) layer during the first stageof the group-III nitride growth. To overcome the effects of amorphous orpolycrystalline SiN_(x) on the growth quality and to facilitate aneffective diffusion barrier layer, formation of a single-crystaldiffusion barrier layer which can be lattice matched to the Si(111)surface is highly desirable.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a multiple-layer buffer,comprising at least two layers of distinct material with sharp materialtransitions and epitaxial alignments between the layers on thecrystalline silicon substrate and between the bottom layer of the bufferwith the crystalline silicon substrate, to resolve the issue of theinter-diffusion between the interfaces of AlN/Si, GaN/Si, or InN/Si.

It is a further object of this invention to provide a single-crystalAlN/Si₃N₄ double buffer layer with coincident lattice conditions on aSi(111) substrate that can alleviate the problems of lattice mismatchand interdiffusion, thereafter inducing high-quality heteroepitaxialgrowth.

According to abovementioned objects, the present invention provides astructure for resolving the issue of auto-doping, resulting from Al/Si,Ga/Si, or InN inter-diffusion when grown with a group-IIII nitridebuffer layer. The structure comprises a Si(111) substrate that surfacehas been reconstructed by in-situ thermal annealing to remove theremained thin oxide layer and to prepare clean and smooth siliconsurface at high temperature. Then, the key feature of the presentinvention, a multiple-layer buffer layer is formed on the reconstructedSi(111) substrate. The multiple-layer buffer includes a single-crystalsilicon nitride layer and a single-crystal AlN layer or other group-IIInitride semiconductor epitaxial layer thereon. Next, the GaN epilayer isgrown on the multiple-layer buffer. The advantage of the presentinvention is that the multiple-layer buffer mechanism can improveheteroepitaxial growth with large lattice mismatch. Furthermore, the 1:2and 5:2 coincident lattices formed at the interface of thesingle-crystal silicon nitride (Si₃N₄)/Si(111) and the interface of thesingle-crystal AlN(0001)/single-crystal silicon nitride (Si₃N₄)respectively can be used to facilitate the multiple-layer buffer forhigh-quality GaN-on-Si heteroepitaxial growth. Thus, the inter-diffusionbetween group-III elements (Al, In, or Ga) and Si can be resolved andthe epitaxial growth quality can be improved.

Moreover, the present invention provides a method for forming agroup-III heteroepitaxial structure on a Si(111) substrate. The keyfeature of the present invention is that the multiple-layer buffer isformed on the Si(111) substrate. The multiple-layer buffer comprises asingle-crystal silicon nitride (Si₃N₄) layer that is formed on theSi(111) substrate by introducing reactive nitrogen-plasma to thereconstructed Si(111) surface in the controlled manner to prevent theformation of amorphous or polycrystalline SiN_(x) layer. Then, anotherbuffer layer is an AlN layer or other group-III nitride layer, which isgrown epitaxially on the single-crystal silicon nitride layer.Similarly, the group-III nitride heteroepitaxial structure isepitaxially grown on the AlN layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a flow diagram showing the steps for growing a group-IIInitride semiconductor heteroepitaxial structure on a Si(111) substratein accordance with a method disclosed herein;

FIG. 2A through FIG. 2D are schematic representations the structure ofvarious stages during the formation of a group-III nitride semiconductorheteroepitaxial structure on a Si(111) substrate in accordance with amethod disclosed herein;

FIG. 3 is a schematic representation the SIMS depth profiles nears thebuffer/substrate and the epilayer/buffer interface regions for sampleswith a single-crystal AlN/Si₃N₄ double buffer layer (a) and asingle-crystal AlN single buffer layer (b) in accordance with a methoddisclosed herein;

FIG. 4A is a schematic representation the low-temperature PL spectra ofGaN epilayer grown on Si(111) using single-crystal AlN/Si₃N₄ and AlNbuffer layers in accordance with a structure disclosed herein;

FIG. 4B is a schematic representation showing the Arrhenius plots of theluminescence intensities of free exciton (FX) grown on a single-crystalAlN/Si₃N₄ double buffer layer and neutral-donor-bound exciton grown onsingle-crystal AlN single buffer layer in accordance with a structuredisclosed herein; and

FIG. 5 is a schematic representation showing the room temperature Ramanspectra taken from the GaN epilayer grown on Si(111) substrate usingsingle-crystal AlN/Si₃N₄ and AlN buffer layers in accordance with astructure disclosed herein.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Some sample embodiments of the invention will now be described ingreater detail. Nevertheless, it should be recognized that the presentinvention can be practiced in a wide range of other embodiments besidesthose explicitly described, and the scope of the present invention isexpressly not limited except as specified in the accompanying claims.

According to the present invention is to provide a method and a stackedbuffer structure for improving the inter-diffusion issue that occursbetween AlN/Si, GaN/Si, and InN.

Group-III nitrides on silicon heteroepitaxy have recently demonstratedto be a viable alternative for growing high-quality group-III nitridefilms for optoelectronic, electronic, and surface acoustic wave deviceapplications. Besides the availability of lager size (up to 12 inch indiameter), low cost, and excellent crystal quality of Si substrates, Sialso possesses excellent material properties such as doping properties(amphoteric type and high carrier concentration), cleavability, goodthermal conductivity (about 3 times larger than that of sapphire), andmature processing techniques. These advantages of Si substrates can openup many novel applications of group-III nitride materials, including thepotential integration of GaN and Si technologies. The reason thathigh-quality GaN-on-Si heteroepitaxy is feasible that is due to thepossible lattice matching of hexagonal wurtzite epitaxial films anddiamond Si(111) crystal faces.

The stacked buffer layer consists of constituent layers, which can formcoincident lattices at layer/layer and layer/substrate interfaces. Inthe preferred embodiment of the present invention, the buffer layercomprising at least two layers of distinct material with sharp materialtransitions and epitaxial alignments between the layers and between thebottom layer of the buffer layer and the Si(111) substrate. For the caseof GaN-on-Si(111) heteroepitaxy, the present invention utilizes the 1:2and 5:2 coincident lattices formed at the β-Si₃N₄ (0001)/Si(111) and AlN(0001)/β-Si₃N₄(0001) interfaces respectively to facilitate the doublebuffer layers for high-quality GaN-on-Si heteroepitaxial growth. Byusing this buffer technique, the present invention can resolve the issueof auto-doping, resulting from Al/Si inter-diffusion when grown with asingle AlN (0001) coincident buffer. As the result, the epitaxialquality of GaN film is also significantly improved.

GaN on Si(111) heteroepitaxy constitutes a large +20.4% in-plane latticemismatch (≡(α_(Si)-α_(GaN))/α_(GaN); α_(GaN(0001))=3.189A;α_(Si(111))=3.840A) and large thermal expansion mismatch. Fortunately,by using a buffer layer with coincident lattice conditions can alleviatethe lattice mismatch. For example, a 5:4 lattice coincidence between AlN(0001) (α_(AlN(0001))=3.112A) and Si(111) can reduce the latticemismatch from 23.4% (tensile strain) to an effective lattice mismatch of−1.3% (compressive strain). As a result, two-dimensional smoothepitaxial growth mode was found to be possible due to the reducedstrain.

The growth processes were conducted in a molecular-beam epitaxy (MBE)apparatus equipped with a radio frequency (RF) nitrogen plasma source.The base pressure in the MBE growth chamber was about 6*10⁻¹¹ Torr.High-purity Ga and Al metals were used for the conventional effusioncells. Nitrogen gas (N₂) was purified through a nitrogen purifier beforefed into the radio-frequency (rf) plasma source. Nitrogen plasma wasgenerated under the same conditions during all the growth processes. TheRF power is about 450 watt and the nitrogen flow rate of the nitrogen isabout 0.5 sccm. Three-inch Si(111) substrate (boron-doped p-type) waschemical etched before loading into the MBE chamber. The Si(111)substrate was further thermally annealed in situ to remove the remainedthin oxide layer and to prepare a clean and smooth silicon surface athigh temperature. The Si(111) substrate prepared by this process showeda clear (7*7) surface reconstruction, confirmed by the reflectionhigh-energy electron diffraction (RHEED) pattern at about 800° C.

Furthermore, the RHEED pattern indicates a high-quality and smoothreconstructed Si surface prior to the growth process. The substratetemperature was calibrated by observing the (7*7) to (1*1) phasetransition of Si(111) surface at 875° C. In the present invention, twodifferent buffer-layer systems for GaN growth on Si (111) substrateswere prepared for comparison. Both samples consist of an AlN bufferlayer with a thickness of 30 nm. The only difference is that one samplecontains a single-crystal β-Si₃N₄ layer [1.5 nm-thick layer with anabrupt interface with the Si(111) substrate, as confirmed bytransmission electron microscopy (TEM)] before the AlN layer growth.Single-crystal silicon nitride layer can be formed by nitridation of theSi(111) surface under the reactive nitrogen plasma for 30 sec at asubstrate temperature of about 900° C. The 30 nm thick AlN buffer layerswere grown epitaxially at 820° C. with a growth rate of 0.12 um/hr.Furthermore, the 240-nm-think GaN epitaxial layers were grown on thebuffer layers at a lower substrate temperature (720° C.) with a growthrate of 0.08 um/hr. After the MBE growth, the grown GaN surface (coolingto 500 to 600° C.) showed a (2*2)-reconstruction pattern under thenitrogen flux, indicating a Ga-polar film. The Ga-polar GaN film isknown to have better structural and optic properties

FIG. 1 shows a flow chart of the method for forming a double bufferlayer on the Si(111) substrate, wherein, FIG. 1 divided into FIG. 1A andFIG. 1B to show the formation process of double buffer layer on theSi(111) substrate. Step 1 illustrates the Si(111) substrate that werethermally annealed in situ to remove the remained thin oxide layer andto prepare a clean and smooth silicon surface at high temperature. TheSi(111) substrate prepared by the process showed a clear (7*7) surfacereconstruction, that can be confirmed by the RHEED pattern at about 800°C. Step 2 illustrates the reactive nitrogen-plasma is introduced to thesurface of the reconstructed Si (111) substrate in a controlled mannerto form the single-crystal silicon nitride (Si₃N₄) diffusion-barrierbuffer layer by nitridation of the surface of the Si (111) substrate.The surface of single-crystal Si₃N₄ diffusion-barrier buffer layerformed in step 2 is terminated by nitrogen surface adatoms. Step 3illustrates the process for forming Al pre-deposition atomic layer onthe single-crystal nitrogen-terminated Si₃N₄ diffusion-barrier bufferlayer. Then, a thermal annealing process is performed to the Alpre-deposition atomic layer to form an AlN monolayer on thesingle-crystal Si₃N₄ diffusion-barrier buffer layer without the reactivenitrogen species (step 4). Next, an epitaxial AlN buffer layer is formedon the single-crystal. Si₃N₄ diffusion-barrier buffer layer byperforming an AlN epitaxial growth process on the AlN monolayer (step5). Finally, the GaN film with a Ga-polarized surface or a group-IIInitride semiconductor heteroepitaxial structure is grown by an epitaxialgrowth method on the single-crystal AlN buffer layer (step 6).

Then, referring to FIG. 2A, the Si(111) substrate 10 is initiallytreated by in-situ annealing process or ex-situ wet etching process(such as etching by an HF solution) to remove the remained thin oxidelayer and to prepare a clean and smooth silicon surface. The Si(111)substrate 10 prepared by the thermal annealing process shows a clear(7*7) surface reconstruction, and is confirmed by the reflectionhigh-energy electron diffraction (RHEED) pattern at about 800° C. TheRHEED pattern can be used to indicate a high-quality and smoothreconstructed Si(111) substrate surface prior to the growth processes.The substrate temperature is calibrated by observing the (7*7) to (1*1)phase transition of Si(111) surface at 875° C.

Next, the key feature of the present invention is that thediffusion-barrier buffer layer 12 such as a single-crystal siliconnitride layer is formed by nitrogen-plasma nitridation of the surface ofSi(111) substrate 10. The nitridation process is performed by exposingthe surface of Si(111) substrate 10 to the reactive nitrogen plasma forabout 30 seconds at a substrate temperature of about 900° C. Exposuretime is critically controlled to prevent the unwanted formation ofamorphous or polycrystalline SiN_(x) layer. In the present invention,the single-crystal silicon nitride [β-Si₃N₄(0001)] layer 12 can also beformed on the Si(111) substrate 10 by introduction of reactivenitrogen-containing species including NH₃ while the surface of Si (111)substrate 10 held slightly higher than the (7*7) to (1*1) phasetransition temperature. A single-crystal (4*4) surface-reconstruction(alternatively, the “(8*8)”-reconstruction in terms of the Si (111)substrate 10 lattice parameter) usually forms after such controllednitridation process.

The RHEED pattern shows the “(8*8)”-reconstructed surface afterintroducing the reactive nitrogen plasma to the surface of Si(111)substrate at the substrate temperature of 900° C. for about 30 seconds.The RHEED pattern shows that are two different ordering onβ-Si₃N₄(0001). One ordering corresponding to the topmost“(8/3*8/3)”-ordered nitrogen adatoms and the other corresponds to the“(8*8)” lattice periodicity. In the previous scanning tunnelingmicroscopy experiments, the “(8*8)” ordering was confirmed to be thereconstruction unit cell of the β-Si₃N₄(0001) surface.

Then, the growth of AlN buffer layer 16 in the double buffer layersystem was started on the nitrogen-terminated Si₃N₄ reconstructionsurface as shown in FIGS. 2B and 2C. After 15 seconds of Alpre-deposition process, the Al atomic layer 14 is formed on the surfaceof single-crystal Si₃N₄ layer 12. Then, a single-crystal AlN bufferlayer 16 stacked on the single-crystal Si₃N₄ layer 12 is formed first byperforming a thermal annealing process to the AlN pre-deposition atomiclayer 14. The AlN (0001)-(1*1) ordering appears in the streaky RHEEDpattern after the thermal annealing step. The RHEED pattern indicatesthat Al atoms are bounded with the topmost N adatoms of thesingle-crystal Si₃N₄ layer 12 and the surface is very smooth. It shouldbe noted that the reciprocal space periodicities along the bulkβ-Si₃N₄(0-110) 12 and AlN(0-110) 16 directions are 4:5 and thiscondition can be confirmed in the RHEED pattern. Then, an AlN bufferlayer 16 is grown by an epitaxial growth method on the single-crystalSi₃N₄ layer 12. In addition, it is also possible that the second bufferlayer stacked on the single-crystal Si₃N₄ diffusion-barrier buffer layer12 is a GaN layer or InN (indium nitride) layer.

In order to compare the effects of single-crystal AlN single buffer andAlN/Si₃N₄ double buffer layer on the grown film quality, the AlN bufferlayer 16 with an identical thickness about 30 nm were grown epitaxiallyon two Si substrates with and without the single-crystal Si₃N₄ layer 12at a substrate temperature of 820° C. with a growth rate of 0.12 um/hr,as shown in FIG. 2D. Moreover, the GaN epitaxial layers 20 with anidentical thickness about 240 nm were grown on the single-crystal AlNbuffer layers 16 of these two samples at a lower substrate temperaturewhich is about 720° C. with a growth rate of 0.08 um/hr. After the MBEgrowth, the grown GaN surface after cooling to 500 to 600° C. shows a(2*2)-reconstruction pattern under the nitrogen plasma flux.

As referring to FIG. 2D, the present invention provides a light-emittingdevice structure with a double buffer layer to resolve the issue of theauto-doping, resulted from the inter-diffusion of Al/Si and Ga/Si whengrown with a single AlN(0001) coincident buffer. The light-emittingdevice provides a Si(111) substrate 10 with in-plane lattice constant of3.84 angstroms. The key feature of the present invention is a doublebuffer layer on the Si(111) substrate 10. In the preferred embodiment ofthe present invention, the double buffer layer can improve thelight-emitting efficiency for the light emitting device, wherein thedouble buffer layer includes a single crystal silicon nitride layer(Si₃N₄) (0001) 12 with in-plane lattice constant 7.61 angstroms, and theAlN(00001) layer 16 with in-plane lattice constant 3.112 angstroms.

In addition, the present invention shows a streaky RHEEED pattern after10-min AlN growth at 840° C. and indicates that the resulting epitaxialAlN buffer layer 16 has a smooth surface and is of high film quality. AGaN epilayer grown on the single-crystal Si₃N₄/AlN double buffer layerby MBE also has a smooth surface morphology and high crystalline qualityas demonstrated by the in-situ streaky RHEED pattern and is confirmed byex-situ X-ray diffraction (XRD) and atomic force microscopy (AFM)measurements. Herein, the possible overgrown heteroepitaxial structureon top of the double buffer layer includes a group-III nitridesemiconductor single epitaxial layer or group-III nitride semiconductorheteroepitaxial multiple-layer. From the in-situ RHEED and ex-situ XRDmeasurements, the present invention can determine that 1:2 and 5:2coincident lattice interfaces are formed at β-Si₃N₄(0001)/Si(111) andAlN(0001)/β-Si₃N₄(0001) interfaces, respectively.

Furthermore, the following epitaxial orientation relationships are foundby the RHEED and XRD studies: β-Si₃N₄(0001)||Si(111); β-Si₃N₄[0{overscore (1)} 10]||Si[11 {overscore (2)}]; β-Si₃N₄[2 {overscore (1)}{overscore (1)} 0]||Si[{overscore (1)} 10] and AlN(0001)||β-Si₃N₄(0001);AlN[0 {overscore (1)} 10]||β-Si₃N₄[0 {overscore (1)} 10]; AlN[2{overscore (1)} {overscore (1)} 0]||β-Si₃N₄[2 {overscore (1)}{overscore(1)} 0]. Thus, the GaN/AlN/β-Si₃N₄ c-axis is perpendicular to thesurface of Si(111) substrate. It is tempting to perform heteroepitaxy ofGaN on the Si(111) substrate using a single β-Si₃N₄ buffer layer(without the AlN buffer layer). However, the present invention confirmsthat the resulting growth is rough and polycrystalline in the firstgrowth stage as indicated by the spotty RHEED pattern during the initialGaN film growth. Therefore, the double buffer layer approach can yieldbetter interface properties between epilayer and buffer layer.

The GaN epilayer grown under the same growth conditions using a singleAlN buffer layer also shows a similar high-quality RHEED pattern.Therefore, in order to compare the influences of these different bufferlayers, secondary-ion mass spectroscopy (SIMS), XRD, photoluminescence(PL), and Raman scattering measurements were conducted to compare thestructural and the optical properties of these GaN films with andwithout the single-crystal β-Si₃N₄ buffer layer.

Firstly, the impurity distribution in the growth direction can bedetected by SIMS. In order to investigate the auto-doping effects whileGaN grown on the Si(111) substrate, the present invention focus on Aland Si ion signal depth profiles in the GaN/AlN and AlN/Si interfaceregions. The SIMS spectra were obtained by using a 7.7 keV Cs+ primarybeam and were used to probe Al and Si depth profiles in GaN films grownon Si(111) substrate using two buffer layer systems. The depth zeropoints of each SIMS spectrum are set at the top of Si(111) substrates.Focus on the Si depth profiles in AlN buffers and GaN epitaxial layers,the magnitudes of the Si ion signal are indicated by the solid arrows inFIG. 3. From the SIMS spectra, the magnitudes of Si impurities in theAlN buffer layer and the GaN film using a single-crystalβ-Si₃N₄(0001)/AlN(0001)/Si(111) double buffer layer is about one orderof magnitude lower than that using an AlN single buffer layer.

Not only the single-crystal Si₃N₄ layer inhibits the Si diffusion intothe AlN and GaN layers, it also prevents the Al diffusion into thesilicon substrate during the high temperature growth of AlN bufferlayer. And, the magnitude of the Al ion signal in Si(111) substrate isalso about one order of magnitude lower than that grown without thesingle-crystal silicon nitride diffusion barrier layer. Therefore, theSIMS spectra show that single-crystal Si₃N₄ layer inhibits Si diffusioninto the AlN and GaN layer and Al diffusion into the silicon substrateeffectively during the AlN high-temperature growth and the sequentialGaN epitaxial growth stage.

FIGS. 4A and 4B show the comparison of the optical properties of GaNfilms that grown on different buffer layers. The low-temperature (6.7 K)PL spectra indicate that the GaN film grown on single-crystalAlN/Si₃N₄/Si(111) has a smaller full width at half maximum (FWHM) ofneutral-donor-bound excition (D∘X) near-band-edge luminescence peak thanthat of film grown on AlN(0001)/Si(111). The decrease in the FWHM valueof PL peak (12 meV vs. 20 meV) is consistent with the deduction ofdislocation density (7*10⁸ cm⁻² vs. 1.1*10⁹ cm⁻²) measured by AFM,confirming a significant improvement in the epilayer crystallinequality. The inset in FIG. 4A displays the main luminescence peakposition in the PL spectra of GaN grown on single-crystalAlN/Si₃N₄/Si(111) at different temperatures, indicating that thedominant PL peak changes to the free exciton (FX) emission at increasingtemperatures (higher than 70 K, 70k_(B)T˜the localization energy E_(loc)of neutral Si donor). In contrast to this behavior, for the AlNsingle-buffer layer sample, the D∘X peak can be followed up to the roomtemperature. This observation is consistent with the SIMS results; i.e.,the GaN film grown on the single-crystal AlN/Si₃N₄/Si(111) contains muchless Si impurities.

FIG. 4B presents the Arrhenius plots of the luminescence intensities ofFX in GaN grown on the single-crystal double buffer layer and D∘X in GaNgrown on the single buffer layer. From the Arrhenius plots, theactivation energy of FX (E_(x)) in the GaN grown on the single-crystalAlN/Si₃N₄/Si(111) was obtained by fitting the thermal activationrelation is about 25 meV, in good agreement with the reported value forFX in undoped GaN. Furthermore, the activation energies of thenon-radiative recombination of D∘X in GaN grown on the single-crystalAlN/Si₃N₄/Si(111) can be fitted by using two thermal activation energies(E_(a1) and E_(a2)). The obtained values of E_(a1) and E_(a2) correspondwell to the known localization energy (E_(loc)˜6 meV) and donor bindingenergy (E_(D)˜29 meV) of Si impurities in GaN.

The present invention performed Raman scattering measurements to comparethe crystal qualities of GaN epitaxial layers grown on Si (111) by usingdifferent buffer layer systems. FIG. 5 displays typical non-polarizedRaman spectra in logarithmic intensity scale collected in backscatteringgeometry along the GaN c ([0001]) axis (along the growth direction)using the 514.5 nm radiation of an Ar+ ion laser as a light source,including the dominant phonon peak from Si substrate near 520 cm⁻¹. Thephonon bands near 568 cm⁻¹ in each Raman spectrum that are GaN E₂ bands.Besides, the A₁ (LO) band near 735 cm⁻¹ is observed only in the GaN filmgrown on the single-crystal double buffer layer and it represents thatthis GaN film has a lower carrier concentration. According to theprevious investigation, the ratio of A₁ (LO) to E₂ Raman intensity is ˜3for the undoped GaN films. The present invention measures the A₁(LO) toE₂ intensity ratio of the GaN film grown on the single-crystal doublebuffer layer is about 3.3 (only slightly larger than 3), indicating thecarrier concentration is nearly as low as an undoped GaN film. This isconsistent with the SIMS spectra, which show a lower Si concentration inthe GaN film grown on Si(111) using a single-crystal AlN/Si₃N₄ doublebuffer layer.

Although specific embodiments have been illustrated and described, itwill be obvious to those skilled in the art that various modificationsmay be made without departing from what is intended to be limited solelyby the appended claims.

1. A method for forming a semiconductor structure, said methodcomprising: providing a crystalline silicon substrate having a bufferlayer thereon, wherein said buffer layer comprising at least two layersof distinct material with sharp material transitions and epitaxialalignments between the layers and between the bottom layer of saidbuffer layer and said crystalline silicon substrate; and forming agroup-III nitride semiconductor structure on said buffer layer.
 2. Themethod according to claim 1, further comprising performing a surfacereconstruction process to said crystalline silicon substrate.
 3. Themethod according to claim 2, wherein said surface reconstruction processcomprises a thermal annealing in ultrahigh vacuum (UHV).
 4. The methodaccording to claim 2, wherein said surface reconstruction processcomprises an in-situ hydrogen-plasma cleaning process.
 5. The methodaccording to claim 2, wherein said surface reconstruction processcomprises an ex-situ wet etching process.
 6. The method according toclaim 1, wherein said forming said buffer layer comprises: forming asingle-crystal silicon nitride layer on a silicon (111) substrate; andforming a group-III nitride layer on said single-crystal silicon nitridelayer.
 7. The method according to claim 6, wherein said forming saidsingle-crystal silicon nitride layer comprises performing anitrogen-plasma nitridation to said silicon (111) substrate.
 8. Themethod according to claim 6, wherein said forming said single-crystalsilicon nitride layer comprises performing a thermal nitridation to saidsilicon (111) substrate.
 9. The method according to claim 6, whereinsaid forming said single-crystal silicon nitride layer comprisesperforming a chemical vapor deposition to said silicon (111) substrate.10. The method according to claim 6, wherein said forming said group-IIInitride layer comprises: performing an aluminum pre-deposition processto said single-crystal silicon nitride layer terminated by nitrogensurface adatoms without introducing reactive nitrogen species to form analuminum pre-deposition atomic layer on said single silicon nitridelayer; performing a thermal annealing process to said aluminumpre-deposition atomic layer to form a single-crystal aluminum nitridemonolayer on said single-crystal silicon nitride layer; and performingan aluminum nitride epitaxial growth process to said single-crystalaluminum nitride monolayer to form said group-III nitride layer on saidsingle-crystal aluminum nitride monolayer.
 11. The method according toclaim 1, wherein said group-III nitride semiconductor structure isformed by chemical vapor deposition method.
 12. The method according toclaim 1, wherein said group-III nitride semiconductor structure isformed by molecular beam epitaxy method.
 13. The method according toclaim 1, wherein said group-III nitride semiconductor structure is agroup-III nitride single layer.
 14. The method according to claim 1,wherein said group-III nitride semiconductor structure is a group-IIInitride multiple-layer structure.
 15. The method according to claim 1,wherein said group-III nitride semiconductor structure is a galliumnitride epitaxial layer.
 16. A method for growing a group-III nitridesemiconductor heteroepitaxial structure, said method comprising:providing a silicon (111) substrate; performing a nitrogen-plasmanitridation process to said silicon (111) substrate to form asingle-crystal silicon nitride layer on said silicon (111) substrate;performing an aluminum pre-deposition process to said single-crystalsilicon nitride layer terminated by nitrogen surface adatoms withoutintroducing reactive nitrogen species to form an aluminum pre-depositionatomic layer on said single-crystal silicon nitride layer; performing athermal annealing process to said aluminum pre-deposition atomic layerto form a single-crystal aluminum nitride monolayer on saidsingle-crystal silicon nitride layer; performing an aluminum nitrideepitaxial growth process to said single-crystal aluminum nitridemonolayer to form an aluminum nitride epitaxial buffer layer on saidsingle-crystal silicon nitride layer; and forming a group-III nitridesemiconductor heteroepitaxial structure by epitaxial process on saidaluminum nitride epitaxial buffer layer.
 17. The method according toclaim 16, further comprising performing a thermal annealing in ultrahighvacuum to said silicon (111) substrate to form a reconstructed silicon(111) surface.
 18. The method according to claim 16, further comprisingperforming an active hydrogen plasma cleaning process to said silicon(111) substrate to form a clean and smooth silicon (111) substrate. 19.The method according to claim 16, further comprising performing anex-situ wet etching process to said silicon (111) substrate to form aclean and smooth silicon (111) surface.
 20. The method according toclaim 16, wherein said performing a nitrogen-plasma nitridation processto said silicon (111) substrate to form a said single-crystal siliconnitride layer on said silicon (111) substrate is a thermal nitridationprocess.
 21. A semiconductor structure with a group-III nitridesemiconductor heteroepitaxial structure, said semiconductor structurecomprising: a silicon (111) substrate; a buffer layer on said silicon(111) substrate, said buffer layer having at least two layers ofdistinct material with sharp material transitions and epitaxialalignments between the layers and between the bottom layer of saidbuffer layer and said silicon (111) substrate; and a group-III nitridesemiconductor heteroepitaxial structure on said buffer layer.
 22. Thesemiconductor structure according to claim 21, wherein said buffer layercomprises a diffusion barrier layer on said silicon (111) substrate. 23.The semiconductor structure according to claim 22, wherein the materialof said diffusion barrier layer is single-crystal silicon nitride(Si₃N₄).
 24. The semiconductor structure according to claim 21, whereinsaid buffer layer comprises an aluminum nitride layer on said diffusionbarrier layer.
 25. The semiconductor structure according to claim 21,wherein said buffer layer comprises a gallium nitride layer on saiddiffusion barrier layer.
 26. The semiconductor structure according toclaim 21, wherein said buffer layer comprises an indium nitride layer onsaid diffusion barrier layer.
 27. The semiconductor structure accordingto claim 21, wherein said group-III nitride semiconductorheteroepitaxial structure is a group-III nitride semiconductor singleepitaxial layer.
 28. The semiconductor structure according to claim 21,wherein said group-III nitride semiconductor heteroepitaxial structureis a group-III nitride semiconductor multiple-layer heteroepitaxialstructure.
 29. A semiconductor structure with a multiple-layer bufferstructure, said semiconductor structure comprising: a silicon (111)substrate; and a multiple-layer buffer structure on said silicon (111)substrate, said multiple-layer buffer structure having at least twolayers of distinct material with sharp material transitions andepitaxial alignments between the layers and between the bottom layer ofsaid multiple-layer buffer structure and said silicon (111) substrate.30. The semiconductor structure according to claim 29, wherein saidmultiple-layer buffer structure comprises a diffusion barrier layer onsaid silicon (111) substrate.
 31. The semiconductor structure accordingto claim 30, wherein the material of said diffusion barrier layer issingle-crystal silicon nitride.
 32. The semiconductor structureaccording to claim 29, wherein said multiple-layer buffer structurecomprises an aluminum nitride layer on said diffusion barrier layer. 33.The semiconductor structure according to claim 29, wherein saidmultiple-layer buffer structure comprises a gallium nitride layer onsaid diffusion barrier layer.
 34. The semiconductor structure accordingto claim 29, wherein said multiple-layer buffer structure comprises anindium nitride layer on said diffusion barrier layer.